One of the most common uses for military radar systems is discerning the angle of arrival of a signal relative to the radar. The fundamental way in which a radar determines angle of arrival is to use a directional antenna on both transmit and receive. The antenna sidelobes significantly attenuate off axis signals, thereby making the radar most sensitive to signals entering through the mainbeam. The mainbeam signal is then processed to determine its angle of arrival relative to the direction in which the antenna is pointing. Meaningful angle of arrival information is available only for targets located in the mainbeam of the radar antenna. Sidelobe targets, when they are of sufficient strength to overcome the sidelobe attenuation of the antenna, yield erroneous angle of arrival data. A radar used to search for and track targets must therefore have some means for discerning off-axis signals in the sidelobes from those entering the mainbeam.
Typically, three primary techniques for identifying off-axis signals are currently in widespread use. The first is the use of a guard channel or channels having a separate broad beam antenna with more gain than the side lobes of the primary receive channel (in the case of a monopulse radar, the Sum channel is the primary receive channel). When the guard channel signal level exceeds that of the Sum channel, off-axis indication is given. The second is the Track method, where off-axis signals are placed under track, and then ignored when their range and/or Doppler do not match that expected for the target of primary interest. The Track method is used primarily to reject discrete clutter when the target of interest is moving relative to the clutter (i.e., is separated in Doppler relative to that of the discrete clutter), and finds application in Medium Pulse Repetition Rate Frequency (MPRF) radars where the processed Doppler space is ambiguous with the Doppler of both the target signal and sidelobe clutter signal. The final technique, applicable to a monopulse radar, is to compare the detected boresight error to a threshold. When the detected boresight error exceeds the threshold in one or both of the principle angle tracking planes of the radar (e.g., pitch or yaw), an off-axis indication is given. This two channel OR technique is based on the principle that in the sidelobe region, the difference pattern sidelobes typically exceed the sum pattern sidelobes over a significant portion of angle space. The two channel OR techniques works best for off-axis signals located on or near one of the principal angle tracking planes of the radar.
While both the guard channel and Track methods can be highly effective, neither is very appealing in an airborne missile application. In airborne missile applications, both packaging volume and time are in very short supply. Available space to package the radar hardware is generally very limited, and with the high velocities involved with missile applications, there is very little time to search for and acquire targets (modern missiles generally have a terminal sensor which is cued to the general location of the target by either off-board or on-board tracks). Guard channels require additional hardware, while placing off-axis targets under track complicates the on-board computer hardware (in terms of having sufficient throughput to process a potentially large number of extraneous tracks) and slows the radar search process by the time needed to initiate tracks and resolve any range and Doppler ambiguities that may be present with the waveforms in use. With the trend toward the use of millimeter wave seekers in missile applications, the seeker antenna is electrically large, thus having high gain. The near in sidelobes, which cover an important portion of angle space, can be well above isotropic, meaning that the guard channel antenna required to cover the near in sidelobe region must be directional. To cover both the near in and far out sidelobes, multiple guard channels can be required, depending on the characteristics of the seeker antenna.
The two channel OR method of off-axis indication is simple, requiring no additional hardware or calculation beyond that used for normal angle of arrival processing. For planes well off of the principle angle tracking planes, however, off-axis identification degrades compared to that in the principal planes.
It would, therefore, be desirable to provide a technique which addresses at least the above problems with prior art systems.